1. Field of the Invention
This invention relates to polymeric memory media for high density optical data storage. Specifically, this invention relates to a birefringent composite AZO dye polymer and its application to erasable, direct overwrite digital storage (optical disk), analog storage (video disk), cache and waveguide memory, and optical storage of information through birefringence.
2. Description of the Prior Art
A. Media
The advancing technology of optical information storage devices capable of instantaneous optical recording, high capacity data storage, and random-access retrieval requires material (media) that generate high signal to noise ratio (SNR), low defect density level, high sensitivity and stability, and direct read after write (DRAW) capability. Optical recording technology has been divided into: 1) read-only, 2) write-once, and 3) erasable (reversible) media discussed in turn below.
i. Read Only Media
With respect to read-only media, a thin photopolymerizable liquid is cured `in situ` between substrate and the master, and stripped off to effect pattern transfer. The photopolymerizable materials used include multifunctional acrylates with epoxy or vinyl silicone as discussed in J. Van den Broek et al., J. Rad. Curing 11, 2 (1984); and J. Kloesterboer et al., J. Rad. Curing 11, 10 (1984) incorporated herein by reference as are all publications cited below.
ii. Write-Once Media
With respect to write-once media methods, these methods include (i) ablative recording (pit forming dye-polymer film, dye pigment films), (ii) bubble-deformation recording, and (iii) nondeformation recording each of which is discussed below. Ablative recording involves optical energy absorption, in the film of non-AZO dyes as discussed in A. Bell et al., IEEE J. Quantum Electron. 4, 487 (1978); V. Jipson et al., J. Vac. Sci. Technol. 18, 105 (1981), in non-AZO dye-polymer solutions as discussed in D. Howe et al., J. Vac. Sci. Technol. 18, 92 (1981); K. Law et al., Appl. Phy. Lett. 36, 884, (1980), and in polymer composites that produce a pit in the film by a combination of ablation and material flow.
How and Wrobel in J. Vac. Sci. Technol. 18, 92 (1981) investigated a yellow 3,3'-carbonyl bis (7-diethylamino coumarin) ##STR1## in cellulose nitrate polymer binder where the wavelength of the recording light was 488 nm and the reading wavelength 633 nm, at which the medium was transparent. At lower exposure, most areas yielded poorly defined marks whereas in high exposure areas holes were formed. Organic polyester yellow dye in polyvinyl acetate has been used by Law et al., Appl. Phy. Lett. 36, 884 (1980) in ablative optical recording. Polyester yellow dye shown below in polyvinyl acetate can be spin coated onto a reflective substrate. The recording can be done at 457 nm and reading at longer wavelengths. ##STR2## There are obvious disadvantages of this technique. Reciprocity failure occurs at low power and high energy recording as a result of heat flow and mass motion. Localized Spot temperature can exceed 1000.degree. C. which can cause significant decomposition and vaporization of the organic material.
Investigation into the pit formation in dye (pigment) film has been carried out by V. Novotny et al., J. Appl. Poly. Sci. 24, 1321 (1979), J. Appl. Phys. 50, 1215 (1979); and L. Alexandru et al., Polym. Prepar. 25, 305 (1984) using a dye designated as NK 1748, i.e., 3,3'-diethyl-1,2-acetyl-thiotetracyanine shown below produced by Japanese Research Institute for Photosensitizing Dyes Co., Ltd. at 835 nm recording wavelength, and at 950 nm and higher reading wavelengths. ##STR3## Other prominent dyes useful for this mode of recording include hydroxy squarylium, ##STR4## and Disperse Red 11 (produced by ICI), Disperse Blue 60 (produced by DuPont), Disperse Yellow 88 (produced by Kodak), fluorescein, 1,5-diamino anthraquinone, and Rhodamine-B.
The second type of write-once media involves bubble deformation. Bubble deformation optical recording media consist of polymers such as PMMA, poly,4-methylstyrene, polycarbonate, polysulfone, and poly(fluorocarbon) as a spacer between thin absorbing layers such as Ti, Au, Pt and reflective substrates such as Si:B:C alloy.
Nondeformation recording media, the third type of write once media, consist of mainly nonpolymeric inorganic materials such as complex chalcogenide glasses not further discussed here.
iii. Erasable Media
With respect to erasable or reversible optical recording technology, the technology of the present invention, in 1983 a prototype reversible system was first demonstrated based on Te:Te O.sub.2 medium doped with Ge, In or Pb, a 830 nm diode laser utilized as the writing and reading beam, and a 780 nm diode laser used as the erasing beam. N. Akihara et al., SPIE Optical Disk Technology 329, 195 (1982); Ohta et al., U.S. Pat. No. 3,971,874 (1976) discuss this technology. Being easy to fabricate, process, and use in erasable memory applications, organic/polymer materials have been widely investigated for the last several years. These reversible media materials have been known as photochromics, phase-change ablative-dye polymers, and dye composites. Some of these are discussed in detail below.
Photochromic material based optical recordings as a means of holographic data storage have been studied by Plessey Research as shown in Plessey Research Ltd., Annual Report, 66, (1977); and Plessey Research Ltd., Photochromic Data Sheets. The Plessey medium is similar to high resolution photographic emulsion and is of the fulgimide family, molecularly dispersed in polymer binder. This fulgimide photochromic system functions as shown below where UV light is used to generate active photochromics, thereby creating an image. An Ar laser is used to erase the image. ##STR5## Thioindigo dyes (non-AZO) have been extensively investigated as the working substance of photochromic materials in optical data storage devices as discussed in G. Haucke et al., J. Prakt. Chem. 321 (6), 978-986 (1979); C. Kages et al., Ber. Bunsenges. Phys. Chem. 86, 716 (1982); and D. Russ, Appl. Opt. 10, 571 (1971). The difficulty with indigo dyes is that they are unstable when exposed to heat and visible light, and insensitive in the IR region.
The theory and mechanism of photochromism relating to optical storage have been studied extensively and are discussed in detail in the following references: G. Brown, (Ed.) "Photochromism" Techniques of Chemistry, Vol. 3, Wiley-Interscience, New York, (1971); R. Hurditch, et al., "Photochromic Material for Optical Data Recording Application", Allen Clark Research Center, Annual Review, page 66 (1977) (holographic information storage); D. Kermisch, "Efficiency of Photochromic Gratings", J. Opt. Soc. Am. 61, 1202-6 (1971) (theory of recording process); W. Tomlinson, "Volume Holography in Photochromic Materials", Appl. Opt. 14, 2453 (1975); W. Tomlinson, "Dynamics of Photochromic Conversion in Optically Thick Samples Theory", Appl. Opt. 15, 821 (1976); and G. Scrivener, "Thick Phase Holograms in Color Center Materials", opt. Comm. 10, 32-6 (1974) (efficiency at wavelengths other than peak absorption). Current photochromic materials record information by absorbing energy.
Advanced research in thioindigo dyes has been directed to the covalent attachment of non-AZO chromophobic dye molecules to a polymer chain as shown by Law in German Patent No. DE 3,007,296 AL (1981). The chemical structure was that obtained by reacting Poly(chloromethyl Styrene) with 7-carboxythioindigo dye; and the second by reacting Poly(diethyleneglycolmethacrylade) with Napthothioindigo dye.
Other examples of erasable memory media include: (i) the work of J. Marotz in J. Appl. Phys. B 3, 181-87 (1985) related to holographic memory storage in Poly(methyl methacrylate) blocks sensitized with titanium-bis cyclopentadienyl-dichloride, (ii) photodichroic memory storage as discussed in N. Borelli et al., Proc. Soc. Photo-Opt. Instrument Engineering, 222, 48 (1980); L. Agrevand et al., Opt. Spectrose. USSR, 48, 442 (1980) and 50, 507 (1981); V. Zhanov et al., Opt. Comm. 30, 329 (1974); and E. Krasnikov et al., Acad. Sci. USSR Phys. Chem. 245, 314 (1979), (iii) photographic media, (iv) photo resist media, (v) photopolymeric media, and (vi) electro-optic media as discussed in R. Bartolini et al., Optical Engineering 15 (2), 99-108 (1976). Apparently, none of these operate on the principle of birefringence.
With respect specifically to holographic and optical storage a number of various recording materials have been used including several mentioned above.
Relatively recent optical storage technologies include (i) electron trapping storage as discussed in T. Lindmayer et al., Laser Focus World, Nov., 119 (1989); (ii) two photon, three dimensional memory storage as discussed in D. Parthenopoylos, Science, 245, 843 (1989); and (iii) magneto-optic storage as discussed in J. Isailovic, "Videodisk and Optical memory Systems", 303, (Prentice Hall) (1985).
iv. Erasable AZO Dye Polymer Media
AZO dye polymer materials have not been reported as used for holography and particularly for polarization holography except by Todorov et al. and Couture et al. The use of methyl orange with poly vinyl alcohol as a polarization holographic material was first reported in 1984-85 by Todorov et al., Applied Optics, 23, 4309 (1984); 23, 4588 (1985); and 24, 785 (1985); and L. Nikolova et al., Optical Acta, 31, 5, 579 (1984). Todorov et al., however, achieved a diffraction efficiency of only 20% at a response time of 40 sec and an index modulation (An) of 0.0009 using their dye polymer system.
Recently, Couture and Lessard, in Applied Optics, 27, 3368 (1988) measured modulation transfer function in thin layers of methyl orange/poly vinyl alcohol and methyl red/poly vinyl. While Todorov, et al. discussed the holographic and birefringent use of AZO dye polymer, Nuyken et al., Makromol Chem. 190, 469 (1989) have only studied the cis/trans isomerization as well as decomposition mechanism and crosslinkings in AZO dye polymer.
Attachment of AZO moiety containing dye to polymer backbone chain has been reported by Ichimura et al., Makromol Chem. Rapid Comm. 10, 5 (1989) as a means to create alignment change in liquid crystal. Furthermore, degenerate four wave mixing experiments have been performed in an AZO dye polymer system by Mailhot et al., SPIE 1183, 268 (1989) involving benzopurpurine 4:B and chrysoidine. Recently a red light sensitive-dye polymer system consisting of Oil Red 0 or Sudan III or Sudan IV in PMMA has been investigated for optical recording by Tomova et al., SPIE 1183, 268 (1989).
B. Direct Overwrite Recording
A desirable feature for all data storage media is the ability to record new information on previously recorded areas without erasing the old information in advance. Such capability was inherent to the magnetic disk and tape technology and has therefore been taken for granted by system developers and other users. The new optical disk technology unfortunately does not have "direct overwrite" capability. Of course, it is not possible to erase information from CD, CD-ROM and WORM media. Magneto-optic media does not permit direct overwrite either. A given track of a magneto-optical disk whose information is no longer needed must be erased during one revolution of the disk before any new information can be recorded on it during a later revolution.
Currently, there are a number of ways around the problem. The first is to change the operating system, so that control of the erasing process is within the disk drive itself. In this way the drive will erase the deleted files during its idle time. This solution is currently unpopular because it requires modification of the existing operating systems. There are no guarantees that in a given environment the drive will have sufficient idle time for all the necessary erasures.
The second solution to the lack of overwritability is to use two lasers and two magnets in each optical head, so that when one pair (laser & magnet) is erasing the old data, another (trailing) pair is recording new information. Considerations of cost and complexity have ruled out this solution as well.
The third state of the art solution involves the design of a bilayer or trilayer magnetic medium with interlayer exchange coupling. Such media must work with two permanent magnets, one of which is responsible for recording on the storage layer and the other in charge of erasing the assisting layer. (In trilayer exchange coupled media, the first layer is the storage layer and the third layer is the assisting layer. The second layer is a layer with in-plane magnetization whose task is only to smooth out the transition of magnetization between the first and third layers). The laser operates in two levels for recording the two kinds of magnetic domains. When the laser power is high a reverse domain is written in both storage and assist layers. When the laser power is low either nothing is written or, if a domain has previously been written, it will be erased. A complete description of this direct overwrite system is provided in K. Aratani et al., SPIE, Vol. 1078 "Data Storage Meeting, " Los Angeles, California, 258 (1989). The complex structure of the media in this scheme reduces the design flexibility available for simpler media and makes performance compromises unavoidable. The bilayer and trilayer media therefore suffer from a reduced carrier-to-noise ratio (CNR) in comparison with the simpler media that contain only one magnetic layer.
The other erasable optical medium, known as phase-change media, permits direct overwrite. Phase-change media, however, has been beset by a variety of problems (phase-segregation being the most severe).
C. Modulation of Recorded Information
With respect to modulation schemes, all traditional data storage media are essentially binary in amplitude (either domain with "up" magnetization or domain with "down" magnetization). This means that the only possibility for modulation encoding is pulse width modulation (since the amplitude is either +1 or -1, one should change the width of the pulse in order to encode information). For a comprehensive description of modulation codes see A. Patel, "Signal & Error-Control Coding", Chapter 5 in "Magnetic Recording", 2, C. Denin Mee, et al. (Ed.), McGraw Hill, (1988).
It is currently possible to store information on a magnetic or optical disk in a way that every transition (up to down or mark to no-mark) corresponds to more than one bit of information. The following example is helpful to understanding this concept (known as modulation coding) in the context of mark-no-mark storage. If the minimum size of the optical spot (focused by a high NA lens on the disk) is 1 .mu.m, then the easiest way to record data is to create marks which are either 1 .mu.m, 2 .mu.m, 3 .mu.m, etc. in size. Although the minimum mark size must be 1 .mu.m, there is no physical limit on the size of marks that are greater than the minimum length.
Currently, NRZI coding of user data imposes the constraint that all marks should be integer multiples of the minimum mark length. Now, if the user data is first encoded into another stream of zeros and ones, and if there are certain constraints on this stream (for instance, minimum number of zeros between successive ones is two and a maximum number of zeros between successive ones is seven, as in the 2,7 code) then it is possible to create marks whose length are no longer integer multiples of 1 .mu.m. In this way the 2,7 code has been able to increase the capacity of the storage media by about 50%. Other modulation codes can provide even more flexibility provided that the medium has enough freedom from jitter because the drawback of modulation code is always the reduced window in time that is available to the system for searching and locating the position of the mark boundary (i.e., transition).
The above discussed methods and materials have shown only limited optical efficiencies. Likewise, current optical disk technology and recording techniques have important limitations because of deficiencies in the storage medium. A material which is birefringent and a method of processing which overcomes the problems of low efficiency, short storage life, difficulty in fabrication, and that is useful in a broad range of optical storage applications and allows for direct overwrite recording would be of great benefit.